Welding unit with miniaturized laser beam

ABSTRACT

The invention provides a welding device ( 10 ) using a power laser beam ( 22 ), the device ( 10 ) being miniaturized, the device ( 10 ) being suitable for being moved very close to the surface ( 52   a ) of the part ( 52 ) to be welded and of reaching welding zones that are difficult to access, but without vapor and particles of molten metal ( 56 ) being capable of penetrating into the inside and dirtying the optical components ( 28, 48, 46 ). Such a device is remarkable in that it includes a feed ( 26 ) of gas under pressure suitable for producing a primary flow of gas ( 50 ) leaving via the front opening ( 20 ) together with the laser beam ( 22 ), and in that it also includes a nozzle ( 60 ) connectable to a source of gas under pressure, said nozzle ( 60 ) producing a secondary flow of gas ( 62 ) sweeping the front opening ( 20 ) transversely, thereby deflecting the primary flow of gas ( 50 ) laterally.

TECHNICAL FIELD OF THE INVENTION

The invention relates to welding by a power laser beam, e.g. by way ofindication, at a power of 500 watts (W) to several kilowatts (kW), andmore particularly it relates to a miniaturized laser beam welding devicesuitable for being brought very close to the surfaces of parts to bewelded and capable of reaching surfaces for welding that are difficultto access because of obstacles situated over them.

STATE OF THE ART AND PROBLEM POSED

It is known to weld metals by means of a power laser beam. To do this, awelding device is used that comprises an enclosure of generally elongateshape in which a laser beam can travel and leave via one end of theenclosure in order to touch the part that is to be welded. The enclosureis light-tight in order to protect the operator from any accidentalpropagation of the laser beam outside the enclosure. The term“light-tight” is used to mean that the enclosure prevents any accidentalexit of the laser beam, e.g. as a result of an optical component beingwrongly adjusted.

Inside the enclosure, the laser beam travels along a path comprising insuccession: a power laser source, a converging optical system, and a“front” opening through which the laser beam can leave the enclosuregoing towards the part to be welded. The converging optical systemconcentrates the laser beam on a focus situated outside the enclosure,the part naturally being placed at said focus in order to be welded. Thelaser source may be a laser generator, or it may be the end of anoptical fiber bringing the laser beam into the enclosure from a remotegenerator. In more elaborate versions, the enclosure may be articulatedand/or include one or more optical mirrors disposed on the path of thelaser beam so as to deflect the laser beam appropriately.

To simplify the use of language, an object is said to be situated “infront of” the front opening when it is outside the device and inregister with the front opening. Conversely, an object is said to be“behind” the front opening when it is on the other side of the frontopening.

During welding, the optical components need to be protected from vaporand droplets of molten metal. The enclosure normally performs thisprotection function, however vapor and droplets of molten metal can passthrough the front opening, penetrate into the enclosure, and dirty theoptical components, in particular those which are close to the frontopening. Various means are used to prevent this:

-   -   reducing the diameter of the front opening to the strict minimum        needed for passing the laser beam;    -   injecting an inert gas under sufficient pressure into the        enclosure, the gas leaving through the front opening and thus        constituting an obstacle to vapor and droplets of molten metal;        and    -   locating the optical components at a distance from the front        opening or locating the front opening at a distance from the        part to be welded.

Patent EP 0 514 235 discloses a device making it possible to weldsurfaces having obstacles overlying them, that welding device comprisingan elongate tubular enclosure, a laser source being disposed at one end,the front opening being disposed laterally at the other end, a mirrorbeing disposed behind the front opening and deflecting the laser beamfrom the laser source towards the opening, the working end of thewelding device being passed between the obstacles and the surfaces to bewelded, specifically the inside wall of a nuclear power station tube. Aninert gas leaves through the front opening and protects the surface thatis being welded from ambient air. That welding device neverthelesspresents the drawback of exposing the mirror to vapor and droplets ofmolten metal because:

-   -   the mirror is very close to the molten metal in the welding        zone; and    -   the pressure of the protective gas is necessarily limited for        reasons of economy and so as to avoid dispersing the molten        metal. It can be seen in the figure that the opening is much        larger than the laser beam passing through it. This enables the        zone that is being welded to be thoroughly surrounded by inert        gas, without creating a jet of gas that could disperse the        molten metal. Unfortunately, the protective gas is traveling        only slowly when it passes through the front opening. It        therefore constitutes only a minor obstacle to vapor and        droplets of liquid metal, and consequently the mirror is quickly        degraded.

The mirror therefore needs to be changed frequently. Such a weldingdevice,is therefore suitable for repairing thin parts that require lowpower only, but it is not suitable for industrial use in a workshop onthick parts using a high power laser.

A first problem to be solved is that of providing a welding head whosefront opening is suitable for being brought very close to the surface tobe welded without the optical components being dirtied by the vapor anddroplets of molten metal and without the molten metal being disturbed,e.g. by a flow of gas passing out through the front opening.

A second problem to be solved is that of providing a welding headsuitable for being passed between the surfaces to be welded and nearbyobstacles situated above the surfaces to be welded.

SUMMARY OF THE INVENTION

To solve the first problem, the invention provides a miniaturized laserbeam welding device, the device comprising a welding head constituted bya wall surrounding a cavity, the wall having an “admission” firstopening and a “front” second opening, a laser beam being suitable forentering into the welding head via the admission opening and leaving itvia the front opening by following a predefined light path. The weldinghead is fed with gas under pressure suitable for producing a “primary”gas flow in the cavity and for leaving the welding head via the frontopening. Such a device is remarkable in that the welding head includes anozzle suitable for being connected to a source of gas under pressure,said nozzle being outside the welding head against its wall. The nozzleproduces a “secondary” flow of gas under pressure transversely sweepingthe space situated immediately in front of the front opening.

The term “transversely” is used to mean that the secondary flow isdirectly parallel to the surface of the front opening. With such adisposition:

a) the primary flow pushes back the vapor and droplets of molten metalheading towards the front opening, thereby preventing said vapor anddroplets from penetrating inside the welding head; and

b) the secondary flow strikes the primary flow perpendicularly at theinstant it leaves through the front opening, thereby deflecting theprimary flow and thus preventing it from reaching the liquid metal whichis being formed a little further away in the vicinity of the focus.

Thus, the vapor and droplets of molten metal cannot penetrate into thewelding head through the front opening and the molten metal of the weldis not disturbed, deformed, or even dispersed by the primary flow ofgas, thus making it possible to place the welding head very close to thesurface to be welded, thereby solving the first problem.

The operator gives the primary flow sufficient force to enable it topush back the vapor and droplets of molten metal that approach the frontopening. Similarly, the operator gives sufficient force to the secondaryflow to deflect the primary flow before it reaches, at least directly,the liquid metal that forms in the vicinity of the focus in front of thefront opening.

Advantageously, the welding head also has a shield positioned in frontof the front opening, said shield being substantially flat and parallelto the wall of the welding head surrounding the front opening, saidshield being made of a rigid material that withstands high temperatures,i.e. niobium, the shield being pierced by a hole positioned on the axisof the front opening, the nozzle being positioned between the shield andthe wall of the welding head. The shield enables the primary andsecondary flows to be channeled transversely to the front opening,thereby improving protection of the molten metal against the primaryflow.

The shield also constitutes a mechanical obstacle to the vapor anddroplets of molten metal heading towards the welding head and around thefront opening, thereby reducing dirtying of the welding head. The shieldacts in particular to form an obstacle to the vapor and droplets ofmolten metal traveling between the front opening and the nozzle, whichdroplets would, without the shield, be likely to be moved back in frontof the front opening by the secondary flow. Thus, and in spite of thepresence of the hole situated between the molten metal and the frontopening, this obstacle effect further improves the integrity of theinside of the welding head against vapor and droplets of molten metal.

The shield is advantageously removable so as to allow it to be cleanedor replaced once it has become dirty.

Also advantageously, the welding head includes a skirt surrounding thefront opening, the nozzle, and the shield, the skirt being open andflaring in front of the shield, the skirt being made of a material thatis thin, flexible, and gas-proof, a gap being left between at least oneedge of the shield and the skirt. The skirt thus co-operates with saidgap to bring the primary and secondary flows of gas over the zone beingwelded, thereby protecting the welding zone from ambient air whileconsuming less gas. The skirt is of a length suitable for coming intocontact with the surface of the part to be welded, said skirt alsoenabling the gas of the primary and secondary flows to be retained abovethe welding zone and consequently simultaneously reducing gasconsumption and oxidation of the part that is being welded.

Also advantageously, the welding head may be provided with a mirrorpositioned immediately behind the front opening and deflecting the laserbeam through 90° towards said front opening. This disposition enablesthe size of the welding head behind the front opening to be reduced,thereby enabling the welding head to be passed between the surfaces tobe welded and a nearby obstacle, with this disposition enabling thesecond problem to be solved.

In particular, the welding head has a thickness E1 between the frontopening and the wall opposite from the front opening that is less than50 millimeters (mm).

DESCRIPTION OF THE FIGURES

The invention will be better understood in view of a detailed embodimentand from the accompanying figures.

FIG. 1 shows a welding head mounted at the end of a welding device fedwith light by an optical fiber.

FIG. 2 shows the welding head.

DETAILED DESCRIPTION

Reference is made initially to FIG. 1. The welding device 10 comprises awelding head 12 constituted by a wall 14 defining a closed cavity 16,the wall 14 being light-tight, the wall 14 nevertheless having an“admission” opening 18 and a “front” opening 20, a laser beam 22 beingcapable of penetrating into the cavity 16 via the admission opening 18and of leaving the cavity 16 via the front opening 20 by following alight path 24. The welding head 12 also has a feed 26 suitable for beingconnected to a source of inert gas under pressure, e.g. argon, said feed26 in this example being a duct passing through the wall 14 so as toopen out inside the cavity 16.

In this example, the admission opening 18 and the front opening 20 areplane, circular, and centered on the light path 24 which constitutes theaxes thereof. The admission opening 18 and the front opening 20 areperpendicular. A mirror 28 is placed in the cavity 16 in the light path24. The mirror 28 deflects the laser beam though 90° so as to direct ittowards the front opening 20. The mirror must naturally be capable ofwithstanding high temperatures. By way of example, it can be made ofZnSe, of copper with a cooling circuit, or it can be of the “dielectric”type.

The welding device 10 also comprises an enclosure 30 of generallyelongate shape with its opposite ends being referenced 30 a and 30 b. Alaser light source 32 is placed at a first end 30 a and produces thelaser beam 22 which travels inside the enclosure 30 along the light path24. The laser beam 22 reaches the other end 30 b having the welding head12 attached thereto, the end 30 b surrounding the admission opening 18so as to allow the laser beam 22 to pass from inside the enclosure 30into the cavity 16 of the welding head 12.

In this example, the laser source 32 is a point source constituted by anoptical fiber which delivers laser light from a remote generator (notshown) via its end 32 a inside the enclosure 30. Also in this example,the end 30 a of the enclosure 30 is constituted by a socket 34 thatsupports the laser source 32, the socket 34 being extended to the otherend 30 b by a straight tube 36, the straight tube 36 being attached tothe socket by, for example, screws (not referenced). The welding head 12is also attached to the end of the straight tube 36. In a preferredembodiment, the connection between the welding head 12 and the enclosure30 is releasable so as to make it possible to combine welding heads 12of different shapes with straight tubes 36 of different lengths.

The welding device 10 includes an optical system 42 on the light path24, which system concentrates the laser beam 22 on a focus 44 in frontof the front opening 20, outside the welding head 12, the focus 44 beingon the light path 24. When the laser source 22 is a point source, theoptical system 42 is a converging system and delivers a real image ofthe laser source 32 at the focus 44. The converging optical system 42comprises two converging lenses. A first lens is a “collimator” lens 46which transforms the diverging laser beam that emerges from the end 32 aof the optical fiber into a parallel beam, and the second lens 48 is a“focusing” lens transforming the parallel beam into a beam thatconverges on the focus 44. This disposition makes it possible to usestraight tubes 36 of different lengths without changing the position ofthe focus 44 relative to the front opening 20 of the welding head 12, solong as the position of the collimator lens relative to the end 32 a ofthe optical fiber and the position of the focusing lens 48 relative tothe welding head 12 both remain unchanged. In this example, the opticalsystem 42 also comprises a prism 49 disposed between the collimator lens46 and the focusing lens 48 in the vicinity of the focusing lens, theprism 49 having a cylindrical portion intersecting the laser beam 22over half of its section, and serving to deflect half of the laser beam22 slightly to a secondary second focus (not referenced) that isslightly offset relative to the focus 44, where this disposition isitself known.

The welding head is described below in greater detail with referencesimultaneously to FIGS. 1 and 2.

The feed 26 produces a “primary” flow of gas 50. The welding head 12 ispositioned above the surfaces 52 a of the parts to be welded 52, thefocus 44 being on said surfaces 52 a, the welding head 12 beingsubjected to displacement parallel to the surfaces 52 a, saiddisplacement being represented by a speed vector 54. Under the heatingeffect of the laser beam 52, the metal melts in the vicinity of thefocus 44 and subsequently solidifies in order to form a welding bead 58.The liquid metal that forms in the vicinity of the focus 44 duringwelding is referenced 56. The liquid metal gives off vapor and dropletsthat might pass through the front opening 20, penetrate into the cavity16 of the welding head 12, and touch the mirror 28 which is immediatelybehind the front opening 20. It will be understood that if the intensityof the primary flow 50 is increased in order to prevent the vapor anddroplets of liquid metal 56 penetrating into the cavity 16, the primaryflow 50 will reach the liquid metal 56 at a speed that is too fast,thereby causing it to be deformed or even dispersed, and thus deformingthe welding bead 58 that is being formed.

The welding head 12 has a nozzle 60 suitable for being connected to asource of gas under pressure (not shown), said nozzle being positionedto deliver a “secondary” flow of gas 62 passing in front of the frontopening 20 transversely relative thereto. The secondary flow 62 sweepsthe entire surface of the front opening 60 but without penetratinginside the cavity 16 through said front opening 20. The secondary flow62 thus collides with the primary flow 50 substantially perpendicularlyto the light path 24 in front of the front opening 20, the secondaryflow 62 thus deflecting the primary flow 50, which can thus no longerarrive directly on the liquid metal 56, which it would disperse. Theterm “front wall” 64 is applied to the outside surface of the wall 14 ofthe welding head situated around the front opening 20. In practice, thefront wall 64 is planas and is perpendicular to the light path 24 whenthe light passes through the front opening 20. The nozzle 60 is fixedagainst the front wall 64 but is eccentric relative to the front opening20. The nozzle 60 nevertheless points towards the front opening 20 andproduces the secondary flow 62 transversely relative to the frontopening 20 and parallel to the front wall 64.

A thin flat shield 66 is placed in front of the front opening 20extending parallel to the front wall 64, i.e. perpendicular to the lightpath 24, with the nozzle 60 being located between the front wall 64 andthe shield 66, the shield 66 being pierced by a hole 68 centered on thelight path 24, the shield 66 being closer to the front wall 64 than isthe focus 44. The shield is made of a material that withstands hightemperature, for example niobium. Such a shield presents severaladvantages:

-   -   firstly, it channels the secondary flow 62 and the primary flow        50 parallel to the front wall 64, thereby providing the liquid        metal 56 with better protection against the primary flow 50;    -   secondly it constitutes an obstacle to vapor and droplets of        liquid metal 56, thereby serving to keep the welding head itself        becoming dirtied;    -   finally it constitutes an obstacle to vapor and droplets of        liquid metal which would otherwise go between the front opening        20 and the nozzle 60 and could then be deflected towards the        front opening 20 by the secondary flow 62. Thus, by providing an        additional obstacle and in spite of the holes situated on the        light path, the shield 66 further improves the protection of the        optical components, and in particular of the mirror 28 against        vapor and droplets of liquid metal 56.

The shield may be made of for example, niobium, of a metal alloy inwhich niobium is the major component, or a nickel-based superalloy. Thehole 68 and the front opening 20 are preferably projections of eachother relative to the focus 44, thus enabling their dimensions to berestricted to the minimum required for passing the laser beam 22.

A skirt 70 is placed around the laser beam 22 between the front opening20 and the focus 44, one end of the skirt extending up to the front wall64 and forming an opening surrounding the front opening 20, the otherend of the skirt forming a flared opening around the focus 44. The skirt70 is made of a material that withstands heat, in particular fromdroplets of liquid metal 56. The skirt is gas-proof laterally, and alsosurrounds the nozzle 60 and the shield 66, leaving a gap 74 relative tothe edge 66a of the shield 66 opposite from the nozzle 60. The skirt isof a height that is suitable to ensure that, during welding, its flaredend 70 a is flush with the surface 52 a of the part to be welded 52. Theskirt 70 brings the primary and secondary flows 50 and 62 over theliquid metal 56 and keeps them there, causing them to pass via theabove-defined gap 74. The gas retained in this way is effective inprotecting the surface 52 a for welding around the liquid metal 56. Thisgas then leaks out between the skirt 70 and the surfaces 52 a that areto be welded together.

During welding, the welding head is moved, preferably in the samedirection as the secondary flow 62 leaves the nozzle 60. Thus, thecombined primary and secondary flows 50, 62 arrive at slower speedparallel to the welding bead 58 that is being formed, thus avoidingdeforming it.

In practice, the shield is removable so that it can be cleaned orreplaced once it becomes too dirtied by vapor and droplets of liquidmetal. For a welding head that is greatly miniaturized, it can be heldby two screws screwed into the front wall, the nozzle being sandwichedbetween the shield and the front wall.

The skirt 70 is, for example, cut out from closely-woven fiberglasscloth, and its outside face is covered in a silicone elastomer layerthat withstands high temperatures, with these two materials beingcommonly available in trade. Such a skirt is both flexible and resistantto tearing. In addition, the silicone elastomer makes the skirtgas-proof and the fiberglass cloth protects the silicone elastomer fromthe vapor and droplets of molten metal and also from the heat radiation.

The invention enables the welding device to be greatly miniaturized.That is why the Applicant also claims a welding device comprising awelding head of the invention of size E1 along the axis of the frontopening 24 a no greater than 50 mm. For a welding head 12 with a skirt70, this size corresponds in practice to the distance between the flaredopening 70 a of the skirt 70 and the outside face 76 of the wall 14behind the front opening 20. Thus, the welding device 10 enables thesurfaces 52 a of parts 52 to be welded in spite of the presence of anobstacle 78 situated at a distance E1 above the surfaces to be welded 52a.

The present invention has made it possible in particular to provide awelding head that is powered by a 4 kW yttrium aluminum garnet (YAG)laser, while requiring a space of no more than 24 mm above the surfaces52 a to be welded.

1. A miniaturized laser beam welding device, the device comprising: awelding head constituted by a wall surrounding a cavity, the wall havingan “admission” first opening and a “front” second opening, and a laserbeam being suitable for entering into the welding head via the admissionopening and leaving said welding head via the front opening by followinga predefined light path, the welding head being fed with gas underpressure suitable for producing a “primary” gas flow in the cavity andleaving the welding head via the front opening, wherein the welding headincludes a nozzle suitable for being connected to a source of gas underpressure, said nozzle being outside the welding head against its wall,the nozzle producing a “secondary” flow of gas under pressure sweepingthe space situated immediately in front of the front openingtransversely, in which the wall surrounding the front opening isreferred to as a “front” wall, wherein the welding head includes ashield positioned in front of the front opening, the shield beingsubstantially flat and parallel to the front wall, said shield beingpierced by a hole situated in front of the front opening, the nozzlebeing positioned between the shield and the front wall.
 2. A deviceaccording to claim 1, wherein the shield is removable.
 3. A deviceaccording to claim 1 or claim 2, wherein the shield is made of niobium.4. A device according to claim 3, wherein the welding head includes askirt surrounding the front opening, the nozzle, and the shield, theskirt being open and flaring in front of the shield, the skirt beingmade of a material that is thin, flexible, and gas-proof, a gap beingleft between at least one edge of the shield and the skirt.
 5. A deviceaccording to claim 1, wherein the welding head also includes a mirrorpositioned immediately behind the front opening, the mirror deflectingthe laser beam coming from the admission opening towards the frontopening.
 6. A device according to claim 5, the wall including a portionbehind the front opening, wherein the welding head is of thickness nogreater than 50 mm, the thickness being measured between the frontopening and the portion of the wall behind the front opening.
 7. Adevice according to claim 5, further comprising an enclosure ofgenerally elongate shape, a source of laser light being disposed at afirst end of said enclosure, the other end of said enclosure beingattached to the welding head and surrounding the admission opening.
 8. Adevice according to claim 7, wherein the laser light source is a pointsource and further comprising an optical system which gives a real imageof the laser source at a focus in front of the front opening.
 9. Adevice according to claim 8, wherein the laser source is an opticalfiber.
 10. A device according to claim 7, wherein the connection betweenthe welding head and the enclosure is releasable.
 11. A laser beamwelding device comprising: a welding head including a wall defining acavity, the wall having a first opening and a second opening; a lasersource configured to provide a laser beam in the cavity via the firstopening and out the cavity via the second opening; a first gas sourceconfigured to provide a first gas flow in the cavity via a first gasinlet and out the cavity via the second opening; a second gas sourceconfigured to provide a second gas flow via a second gas inlet, saidsecond gas flow being directed so as to collide with the first gas flowoutside said cavity; and a shield positioned outside said cavity and infront of the second opening, the shield being substantially flat andparallel to a front wall defining said second opening, said shielddefining a hole in front of the second opening, the second gas inletbeing positioned between the shield and the front wall.
 12. The deviceaccording to claim 11, wherein said second gas flow is directed parallelto said front wall.
 13. A device according to claim 11, wherein theshield is removable.
 14. A device according to claim 11, wherein theshield comprises niobium.
 15. A device according to claim 11, whereinthe welding head includes a skirt surrounding the second opening, thesecond gas inlet, and the shield.
 16. A device according to claim 11,wherein the welding head includes a mirror configured to deflect thelaser beam coming from the first opening towards the second opening. 17.A device according to claim 11, wherein said laser source comprises anoptical fiber.
 18. A device according to claim 11, wherein said lasersource comprises a laser.
 19. A device according to claim 18, whereinsaid laser is an yttrium aluminum garnet laser.
 20. A device accordingto claim 11, further comprising an optical system configured to providea real image of the laser source at a focus position in front of thesecond opening.